The present invention relates to a wireless transmission device capable of a feedback control in which the transmission output is controlled to a predetermined level.
FIG. 1 is a block diagram showing an internal configuration of a conventional wireless communications device. The wireless communications device includes an antenna 1, SPDT (Single Pole Double Throw) switches 6 and 8, filters 7, 10 and 11, a low-noise amplifier (LNA) 9, a transmit/receive signal processing section 2, a power amplifier 100, and an output power detector 3.
A transmit RF (Radio Frequency) signal generated by the transmit/receive signal processing section 2 is input to the power amplifier 100 via the filter 11. The power amplifier 100 amplifies the transmit RF signal. The transmit RF signal amplified through the power amplifier 100 is output to the antenna 1 via the SPDT switch 8, the filter 7 and the SPDT switch 6, and is transmitted to the outside from the antenna 1. The output signal of the power amplifier 100 is detected by the output power detector 3, and the detection output DETOUT.1 is an input of a transmission output level judgment function 4 of the transmit/receive signal processing section 2. If it judges that the transmission output level is insufficient, a transmission output level controlling function 5 of the transmit/receive signal processing section 2 increases the output level of the transmit RF signal generated by the transmit/receive signal processing section 2. If it judges that the transmission output level is excessive, the transmission output level controlling function 5 decreases the output level of the transmit RF signal. A feedback control is realized as described above to control the level of the signal transmitted from the antenna 1 to an intended level.
FIG. 2 shows an internal configuration of the power amplifier 100 and the output power detector 3 shown in FIG. 1.
The power amplifier 100 consists of multi-stage (three stages in the illustrated example) amplifier. which includes amplifiers 101, 102 and 103, and inter-stage matching networks 104 and 105. The transmit RF (Radio Frequency) signal generated by the transmit/receive signal processing section 2 of FIG. 1 is input to an RF signal input terminal 107 via the filter 11. The RF signal input terminal 107 is connected to the input of the amplifier 101 of the first stage. The output of the amplifier 101 is connected to one end of the inter-stage matching network 104. The other end of the inter-stage matching network 104 is connected to the input of the amplifier 102 of the following stage. The output of the amplifier 102 is connected to one end of the inter-stage matching network 105. The other end of the inter-stage matching network 105 is connected to the input of the amplifier 103 of the following stage (the last stage in the illustrated example). The output of the amplifier 103 of the last stage is connected to an RF signal output terminal 108. The signal output from the RF signal output terminal 108 is output to the antenna 1 via the SPDT switch 8, the filter 7 and the SPDT switch 6, and is transmitted to the outside from the antenna 1. The output of the amplifier 103 is also connected to one end of a detection diode 106 of the output power detector 3. The other end of the detection diode 106 is connected to a detection output terminal 109. The detection diode 106 detects the output power of the power amplifier 100 and outputs the detection results from the detection output terminal 109.
With the configuration shown in FIG. 2, the detection output DETOUT.1 from the detection output terminal 109 exhibits diode characteristics, thereby resulting in a low detection sensitivity for a lower range of the output level of the power amplifier 100. Therefore, in view of the controllability for lower output levels of the power amplifier 100, this cannot be said to be an effective circuit as the controllability of the wireless communications device is poor. Moreover, since the output of the power amplifier 100 is used as the input to the output power detector 3, the detection output DETOUT.1 from the output power detector 3 is influenced by the reflected signal due to the impedance mismatch in the antenna section. Therefore, the detection output is not stable, and the transmission output of the wireless communications device is also not stable.